Upconversion nanoparticles (UCNPs) have recently emerged as a new class of materials with potential applications in a wide-range of fields, such as biosensing, chemical sensing, in vivo imaging, drug delivery, photodynamic therapy and photoactivation. (Zhan, et al. 2011 Acs Nano 5, 3744; Wang, et al. 2005 Angew Chem Int Edit 44, 6054; Achatz, et al. 2011 Angew Chem Int Edit 50, 260; Liu, et al. 2011 Acs Nano 5, 8040; Liu, et al. 2011 J Am Chem Soc 133, 17122; Chen, et al. 2012 Acs Nano 6, 8280; Lim, et al. 2006 Nano Lett 6, 169; Wang, et al. 2011 Biomaterials 32, 1110; Hou, et al. 2011 Adv Funct Mater 21, 2356; Tian, et al. 2012 Adv Mater 24, 1226; Shan, et al. 2011 Adv Funct Mater 21, 2488; Zhang, et al. 2007 J Am Chem Soc 129, 4526; Jayakumar, et al. 2012 Natl Acad Sci USA 109, 8483; Yang, et al. 2012 Angew Chem Int Edit 51, 3125; Yan, et al. 2012 J Am Chem Soc 134, 16558; U.S. Pat. Nos. 7,332,344; 7,790,392; 7,501,092; 8,088,631.)
Upconverting luminescence refers to an anti-Stokes type process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength (e.g., ultraviolet, visible, and near-infrared) than the excitation wavelength. For instance, Lanthanide ion (Ln3+) doped UCNPs are able to absorb near-infrared (NIR) photons and convert such low energy excitation into shorter wavelength emissions. (Haase, et al. 2011 Angew Chem Int Edit 50, 5808.) Utilizing long-lived, ladder-like energy levels of Ln3+, the intensity of anti-Stokes luminescence of UCNPs is orders of magnitude more potent compared with those of conventional synthetic dyes or quantum dots (QDs). (Wang, et al. 2009 Chem Soc Rev 38, 976; U.S. Provisional Appl. No. 61/675,019 by Han, et al.; U.S. Provisional Appl. No. 61/653,406 by Han, et al.; PCT/US13/42555 by Han, et al. filed May 24, 2013.)
Challenges remain, however, that hamper the wide use of UCNPs. For example, a major limitation of the most commonly used Yb3+-sensitized UCNPs is their physically unalterable excitation band centered at 980 nm (the peak absorption of Yb3+ ions), overlapping with the maximum absorption peak of water molecules (FIG. 1). Because cells and tissues withhold 980 nm radiation and concomitantly induce heat damages, this becomes problematic for application of UCNPs in water-rich biological systems. (McNichols, et al. 2004 Laser Surg Med 34, 48; Nam, et al. 2011 Angew Chem Int Edit 50, 6093.) In particular, the heating effect is likely more severe where greater power density and longer-term irradiation are required, such as in single nanoparticle imaging or longitudinally deep tissue imaging.
While extensive research has resulted in continued progress in the modulation of UCNP's emissions, for example, via composing proper dopants/matrix or FRET process, few studies have focused on engineering the excitation of UCNPs. (Tian, et al. 2012 Adv Mater 24, 1226; Wang, et al. 2011 Nat Mater 10, 968; Li, et al. 2008 Adv Mater 20, 4765; Yi, et al. 2011 Chem Mater 23, 2729; Chan, et al. 2012 Nano Lett 12, 3839; Heer, et al. 2003 Angew Chem Int Edit 42, 3179; Zhan, et al. 2011 Acs Nano 5, 3744.) A recent report showed the use of an alternative excitation peak at 915 nm in Yb3+-sensitized cubic phase (a) NaYF4:Ln UCNPs. This excitation peak, however, is still well within the regime of intrinsic absorption of Yb3+ dopants, which partially overlaps with the absorption peak of water. Another report used dye-sensitized UCNPs under 800 nm excitation, in which an antenna dye was used to stimulate the Yb3+—Er3+ upconverting process via the fluorescence resonance energy transfer (FRET) mechanism. (Zou, et al. 2012 Nat Photonics 6, 560.) This FRET-based approach, however, is limited to the organic media with the synthetic dyes susceptible to photo-bleaching. Additionally, the FRET process is restricted by the distance between the organic molecules and the UCNPs.
Thus, un-met needs continue to exist for novel compositions and methods that enable upconverting luminescence with efficient excitation away from peak absorption of water, preferably near a minimum of water absorption.